The present invention relates generally to the field of electrical impedance tomography (EIT), and in particular to new and useful circuits and calibration algorithms or techniques which permit highly precise current waveforms to be produced and introduced to various loads.
This invention was developed for use in electrical impedance imaging (also called electrical impedance tomography and electrical impedance spectroscopy) where the generation of current waveforms is needed for the purpose of diagnosing breast cancer and other disease.
It should be noted that other applications in electrical impedance imaging exist such as defect detection, geological imaging, and process monitoring. Likewise, the invention may be useful for applications other than electrical impedance imaging.
To obtain the data needed to reconstruct an electrical impedance image, current waveforms are applied to a load through electrodes, the voltages that appear on the electrodes are measured, and these data are processed by a reconstruction algorithm to generate a two or three-dimensional image of the interior conductivity and/or permittivity. The current waveforms are typically sinusoids with a frequency in the range of 100 Hz to 10 MHz. In a 32-electrode system, as many as 32 current sources may be used to apply currents to all the electrodes simultaneously. Each set of applied currents is called a current pattern.
The patterns of current that are applied (U.S. Pat. Nos. 4,920,490; 5,588,429; 5,381,333; 5,272,624), methods by which voltages are measured (U.S. Pat. No. 5,544,662) and the algorithms which reconstruct the images (U.S. Pat. Nos. 4,920,490; 5,284,142; 5,351,697; 5,390,110) have been previously described.
The quality of the images produced in impedance imaging depends greatly on the precision of the applied currents. Precision can be defined as the reciprocal of the fractional change in current resulting from a change in load impedance. High precision reflects little change in current while low precision reflects large change in current. The current sources must be able to provide the desired current over the range of load impedances presented by the electrodes. To achieve this precision, the current sources should have an output impedance that is much higher than the load impedances. Here, a new current source is described which improves the precision of the applied currents and expands the frequency range over which currents may be generated. To our knowledge, no current source presently exists which is capable of producing an output over the range of 100 Hz and 1 MHz with output impedances in the tens to hundreds of Megohms. This level of output impedance is required in order to achieve the level of precision necessary in an optimized, applied current, impedance imaging system.